In the field of lubricating oils, additives such as pour point depressants have conventionally been added to lubricating base oils including highly refined mineral oils, to improve the properties such as the low-temperature viscosity characteristics of the lubricating oils (see Patent documents 1-3, for example). Known methods for producing high-viscosity-index base oils include methods in which feed stock oils containing natural or synthetic normal paraffins are subjected to lubricating base oil refining by hydrocracking/hydroisomerization (see Patent documents 4-6, for example).
The properties evaluated for the low-temperature viscosity characteristics of lubricating base oils and lubricating oils are generally the pour point, clouding point and freezing point. Methods are also known for evaluating the low-temperature viscosity characteristics for lubricating base oils according to their normal paraffin or isoparaffin contents.
The purpose of using lubricating oils in internal combustion engines, gearboxes and other mechanical devices is to produce smoother functioning in such devices. Internal combustion engine lubricating oils (engine oils), in particular, must exhibit high performance under the high-performance, high-output and harsh operating conditions of internal combustion engines. Various additives such as anti-wear agents, metal-based detergents, ashless dispersants and antioxidants are therefore added to conventional engine oils to meet such performance demands.
The fuel efficiency performance required of lubricating oils has continued to increase in recent years, and this has led to application of various high-viscosity-index base oils or friction modifiers (see Patent document 7, for example).